One major reason against underground mining on planets is the huge amounts of free ore available to players.
The problem is that everyone can have a huge factory and produce items for next to nothing (with the exception of schematics). This leads to a collapsing market with extremely low prices and a huge oversupply of products.
Each factory owner can depress his prices by simply digging into the ground.
In addition, there is almost no way for factory owners to buy ore on the market (without digging), produce goods, and sell them on the market at a profit.
It is very questionable whether the current system without underground mining on planets can prevent these problems, but at least it will delay the collapse of the market a little...
just a thought,
Cheers ...